Optical pigtail assemblies are used for coupling the output from an optical source, such as a light emitting diode, a diode laser or a diode laser pumped solid-state laser, to an optical waveguide, such as a fiber optic cable or the like. The optical waveguide is positioned within the laser pigtail assembly for maximum optical power transfer from the optical source to the waveguide.
In a typical single mode laser pigtail assembly, a laser source, usually contained in a metal can or housing, is mechanically carried by a base member. A lens, such as a gradient index, plano-convex lens or a spherical lens, is fixedly positioned adjacent to a window in the laser source housing. When using a plano-convex lens, the convex end of the lens is typically positioned towards the laser to focus the output of the laser source. A cap member is mechanically placed over or adjacent to the laser source and the lens. The cap member has a central aperture into which is secured an optical fiber waveguide. The optical fiber can be placed directly into the aperture or, alternately, the fiber can be placed in a ferrule and then secured in the aperture. The optical fiber and the cap member are positioned with respect to the output of the laser source for maximum optical power transfer from the laser source to the optical fiber and then welded together using a precision laser welding tool (e.g., flash welding).
An alternative method of manufacturing such a laser pigtail assembly is to use an adhesive, such as a long-term-curing epoxy, to hold together the various parts. The manufacturing steps are somewhat similar to the previously described process in that the optical fiber is aligned with respect to the laser source for maximum power transfer. In the long-term-curing epoxy process, a layer of epoxy, on the order of 50 microns in thickness, is applied between the parts being bonded. The assembled parts are then set aside until the epoxy is cured.
U.S. Pat. No. 4,969,702 to Anderson describes an improved epoxy process. There, an optical pigtail assembly is manufactured using an ultraviolet (UV) cured epoxy, wherein the thickness of the UV cured epoxy between the bonded elements is on the order of 10 microns and the epoxy is exposed to ultraviolet light after each assembly step to promote curing. In particular, a diode laser is fixedly positioned on a support member; a focusing member, such as a gradient index lens, is fixedly positioned with respect to the diode laser so that the output of the diode laser passes through the focusing member; a cap member having a central aperture therein and a mounting surface normal to the aperture is positioned over the diode laser and the focusing member with the mounting surface in contact with the support member; an optical waveguide, such as a fiber optic cable, is positioned within the central aperture of the cap member; and the cap member and the optical waveguide within the cap member are fixedly positioned with respect to the output of the diode laser passing through the focusing member to provide maximum optical power transfer between the diode laser optical source and the optical waveguide.
Those skilled in the art understand that the dimensional positions of the laser source, the lens and the optical fiber are critical and must be maintained during the manufacturing process. There are at least two techniques which which can be used to achieve these critical tolerances.
One technique is to accurately manufacture the parts required to make the fiber/laser assembly, such that, when the parts are brought together, physical contact is made among the parts. The parts in contact can be secured using a variety of methods, such as laser-welding. The major disadvantage of this scheme is the high tolerance requirements on the components that are used in the assembly.
A second technique is to relax the mechanical tolerances of the components. These components are then brought into close proximity during assembly with gaps between the components to be joined. These gaps are then filled with an epoxy or similar substance. The disadvantage of this scheme is an epoxy can expand or contract during the curing or cooling process and change the accurate placement of the components that are being assembled.
Low volume production is another disadvantage of using ordinary epoxies, since substantial curing time (e.g., 16 hours) is required for each assembly step. In addition, both the epoxy process and the laser welding process require large capital investments. For many epoxy processes, a humidity and vibration controlled room is required while the laser welding process requires expensive laser welding machines. UV-cured epoxy provides some advantages over other methods of assembly, since the thickness of the epoxy layer between adjacent parts is thinner and the bond is generally immune from environmental problems (such as moisture absorption) associated with other kinds of epoxies (such as longer-curing, gyro grade epoxies). However, almost all epoxies have some shrink, creep and expansion effects. Moreover, the parts must be bought very close to each other and positioned very acurately for a UV cured epoxy to work. In addition, stainless steel members are used with the UV epoxy; stainless steel has high thermal conductivity and has a relatively high thermal coefficient of expansion relative to glass. Brass also has the same problem. Finally, if a mistake is made in using an epoxy, the assembly cannot be easily repaired.
What is needed is an optical pigtail assembly which has high optical power coupling efficiency, which has good thermal performance and which can be manufactured using a high output and low cost method that does not require large capital investment.